1. Field of the Invention
The present invention relates to an optical position measuring device.
2. Description of the Related Art
A miniaturized scanning unit for an optical position measuring device suitable for incident light-scanning of an incremental measuring graduation is known from the publication New Dimensions in Position and Angular Measuring Technology by R. Burgschat in F & M 104, 1996, 10, pp. 752 to 756. Inter alia, the scanning device includes an incremental signal scanning arrangement placed on a support substrate. The incremental signal scanning arrangement essentially includes a photodiode array with a multitude of incremental signal detector areas and a centrally arranged light source. Details regarding the structure of the incremental signal scanning arrangement can also be found in DE 195 27 287 A1.
A second photodiode array for generating reference pulses is arranged next to the first photodiode array for generating incremental signals and spaced apart from the first array in the measuring direction. An alternative variation for generating a reference pulse signal in such a scanning unit is moreover known from DE 199 21 309 A1.
Besides position measuring devices with only a single incremental signal scanning arrangement on the side of the scanning unit, a longitudinal measuring system is known from the company publication “Encoder-Kit L” of NUMERIK Jena GmbH with the designation “Kit L-D-03/99”, published in March of 1999, wherein two such incremental signal scanning arrangements are provided, clearly spaced apart from each other, on a common support substrate. A continuous scanning grating for the two incremental signal scanning arrangements is arranged on the underside of the support substrate which faces the scanned measuring graduation. The generation of phase-shifted partial incremental signals from the multitude of the incremental signal detector areas takes place by an appropriate relative arrangement of the incremental signal detector areas with respect to each other.
In systems with only a single incremental signal scanning arrangement in the scanning unit, the resultant sensitivity regarding dirt on the measuring graduation has shown itself to be particularly disadvantageous. The reason for this is a relatively small scanning field on the scale. Faulty measurements are the result, if during the measuring operation the scanning field is occupied by dirt particles.
In connection with the systems with two spaced apart incremental signal scanning arrangements also mentioned above, it is considered to be disadvantageous that only a limited operating temperature range is available because of the different thermal coefficients of expansion of the scale and the scanning unit. This is based on the fact that because of the large distance between the incremental signal scanning arrangements it is no longer assured in case of temperaturerelated position changes that the predetermined phase positions on the various detector elements remain the same. Furthermore, a definite effect on the signal amplitudes, as well as the signal offset, still results in case of dirt on the measuring graduation.